Note: Descriptions are shown in the official language in which they were submitted.
21206~9 -
- A7-19521/A
'
Process for the production of fine granules
The invention relates to a process for the production of fine granules in accordance with
the preamble of patent claim 1.
Various processes for the production of granules are known. US-A-4 578 021 describes a
granulating process in which a supercooled melt provided with seed crystals is pressed
through openings in a perforated hollow roller in which a rotating pressure roller is
provided. The pressed-out drops fall onto a cooling belt and can there solidify or crystal-
lise out. The granules that can be obtained by that process are of a relatively large size
which is determined by the dimensions of the openings in the hollow roller. -~`
In another known process for the production of fine granules, dry powder, powder .
mixtures or also centrifuged and then washed crystallisate is/are mixed with product melt
in a high-speed mixer. The granules formed by crystallisation during that operation are
discharged from the high-speed mixer and then dried andlor cooled in a drier, for example
a moving bed drier or a fluidised bed drier. Although that process is able to yield fine,
low-dust gramlles, the granules are in many cases not abrasion-resistant. In addition, the
application of the process is limited. In particular, in many cases it has hitherto been
impossible to use that process to granulate products having high melting points or having
high viscosities of the product melt, since the product melt does not adequately wet the
powder, or the powder mixture or the crystallisate, in the high-speed mixer. Owing to the
long crystallisation time, the product melt often does not crystallise completely, which
leads to incrustation of the mixer and to lumps which may clog the pipes.
The problem of the invention is accordingly to provide a process for the production of fine
granules by means of which substantially dust-free, stable, completely crystallised
granules or agglomerates can be produced. It is also to be possible to use the process in the
case of products having high melting points andlor having high viscosities of the product
melt. The process is to permit produc~s of mixtures of any number of components tO be - `
granulated in small particle siæs.
~ :~` 2120659
-2-
Those problems are solved according to the invention by a process that comprises the
process steps listed in the second part of patent claim 1. In the process according to the
invention for the production of fine granules a product melt from a melt tank is super-
cooled and then metered or sprayed into a high-speed mixer and there brought into contact
with dry powder, powder mixtures or also with centrifuged and then washed crystallisate
from a silo. During that operation, granules are formed by crystallisation, which are
discharged from the high-speed mixer, subsequently dried and/or cooled in a drier, for
example a moving bed drier or a fluidised bed drier, and then removed from the drier.
Prior to cooling, the product melt is mixed with a solvent, as a result of which its viscosity
is greatly reduced and its wetting capacity in respect of the powder, the crystallisate or the
powder mixtures is substantially increased. The product melt/solvent mixture is then
supercooled preferably to within the metastable range of the solubility curve so that rapid
crystallisation commences in the high-speed mixer. During that operation, the temperature
of the product meltJsolvent mixture is so regulated that the mixture does not boil at the
spray outlet in the high-speed mixer. The viscosity of the product melt is thus markedly
reduced and the wetting capacity of the product melt is greatly improved. Crystallisation
takes place very rapidly in the high-speed mixer and incrustation and lumps are avoided.
The dependent patent claims relate to preferred process variants.
The process according to the invention is explained in more detail hereinafter with refer-
ence to the single Pigure which shows diagrammatically a granuladon system for implem-
enting the process according to the invention.
The granuladon system is indicated as a whole by the reference numeral 1. It comprises a
melt tank 2 from which a product melt M is conveyed by way of a pipeline 3 to a cooler 4
by means of a pump 18. The product melt M is supercooled in the cooler. From the cooler
it passes by way of a pipeline 23 to a high-speed mixer 5. Such a mixer is described, for
example, in company brochure No. wd 10.84 Rec.-d-26.3000 M of Gebruder Lodige
Maschinenbau-GmbH, Elsener Str. 7-9, Postfach 2050, D-4790 Paderborn. The product
melt M is metered or finely sprayed into the high-speed mixer 5 by means of spray
nozzles 20 or distributing tubes and, during that operation, comes into contact with a
powder P, a powder mixture or with a crystallisate that has been centrifuged and then
washed. The powder or powder mixture or crystallisate is stored in a powder silo 6. An
amount weighed out using a weighing device 7 is introduced into the high-speed mixer by
way of a pipeline 22. In the mixer 5, the atomised product melt droplets settle on the
-` 2120~5~
powder or the crystallisate which acts like a crystallisation seed and generally induces
rapid crystallisation. The granu1es formed are conveyed by way of a pipeline 24 to a
drier 8, for example a moving bed drier or a fluidised bed drier, and are there dried and/or
cooled. The dried and/or cooled granules are finally removed from the drier 8 by way of a
pipeline 9.
Before the product melt M is supercooled in the cooler 4, it is mixed with a solvent S
which, by means of a pump 19, is conveyed by way of a pipeline 21 out of a solvent
tank 16 and to a mixing device 17. As a result of mixing the product melt M with the
solvent S, the viscosity of the product melt is greatly reduced, for example from the range
of approximately 1000 to over 100 000 mPas to the range of approximately 30 to
1500 mPas. At the same time, the wetting capacity of the product melt M in respect of the
powder, crystallisate or powder mixture is substantially increased. The product melt/-
solvent mixture is supercooled in the cooler 4, preferably to within the metastable range of
the solubility curve. It is thus ensured that the atomised product melt/solvent mixture crys-
tallises very rapidly in the high-speed mixer S when it comes into contact with the powder,
powder mixture or crystallisate P. The temperature of the product melt/solvent mixture is
preferably so regulated that the mixture does not boil at the spray outlet 20. It will be
understood that the pipeline 23 between the cooler 4 and the high-speed mixer 5 is for that
purpose constructed to be heatable in a regulated manner. For example, the pipeline 23 is a
double-walled pipe in which circulates a heating liquid, the temperature of which can be
adjusted.
In the mixing device 17, product melt M is mixed continuously with the solvent S. In
order to supersaturate the solution, the product melt/solvent mixture is supercooled.
Starting from the solubility equilibrium temperature (at which there exists a saturated
solution) the product meltlsolvent mixture is supercooled by up to 70C. The degree of
supersaturation increases with the degree of supercooling.
.,~
In the regulation of ~he temperature of the supersaturated solution, care is preferably taken
that the ~emperature of the supersaturated solution is kept lower than the boiling tempera-
ture of the solvent. If the product melt/solvent mixture is mixed and transported under
elevated pressure the temperature can be higher, and it is also found that smaller amounts
of solvent S will suffice. A superpressure of from approximately 0 bar to approximately
5 bar is preferably set.
2120659
'4. '
- 4 -
In order to prevent the evaporation of solvent from the product melt/solvent mixture and -
the commencement of crystallisation, which may cause the pipes to become clogged, the
mixing of the solvent S with the product melt is carried out in a closed system, care being
taken that the system is free of pockets of gas.
It is especially advantageous if the mixing device 17, in which the product melt M is
mixed with the solvent S, is a static mixer. Such static mixers are widely used in chemical
synthesis and are described, for example, in an article by Adolf Heierle "Statische
Mischer-Warmeaustauscher", in chemie-anlagen + verfahren 7/89. With regard to the
process according to the invention, the advantages of a static mixer result especially from
the fact that the system is free of pockets of gas. In addition, it has no moving parts, is
easy to clean and is inexpensive. It would, however, also be possible to use a rotor-stator
mixer system instead of a static mixer.
The obtainable size of the granules produced can be altered by varying the ratio of the
amounts of the solvent S and the product melt. The quantitative ratio is preferably so
adjusted that granule sizes of from approximately 0.15 mm to approximately I mm, with a
range of fluctuation of up to approximately i 0.3 mm, are obtained.
In the process according to the invcntion for the production of fine granules, polar
solvents, such as, for example, methanol, isopropanol or water, are preferably used. The
solvent S is recovered when the granules are dried in the drier 8, which, apart from
comprising an inert gas circuit 12 for providing inert gas heated in a heater 11, also
comprises a filter 13, a condenser 14 and a solvent separator 1~ by means of which the
solvent S evaporated off in the drier can be recovered in known manner.
By mixing the product melt with a solvent, the viscosity of the product melt is greatly
reduced, as a result of which many highly viscous product melts can also be granulated in
a high-speed mixer. In addidon, the mixing of the product melt with the solvent also
increases to a marked degree the wetting capacity of the atomised product melt in respect
of the powder, powder mixture or crystallisate, as a result of which the crystallisation time
is greatly reduced. The process according to the invention is accordingly also suitable for
the granulation of product melts of antioxidants, stabilisers or light-protective agents,
which, in the unmixed state, usually have too high a viscosity. The supercooling of the
product melt/solvent mixture ensures spontaneous crystallisation in the high-speed mixer.
As a result, incrustation of the walls of the high-speed mixer is prevented, and the
~:- 2120~9
formation of lumps, which could clog the pipelines, is obviated. The temperature control -
of the process according to the invention is such that undesired crystallisation in the pipe-
lines or in the cooler is largely excluded.
The process according to the invention can be used for powders comprising one or more ~ -
components. That is to say, apart from pure substances, it is also possible to granulate
powder mixtures in fine particle sizes. The components of the powder rluxtures may be
pre-mixed or may be mixed directly in the high-speed mixer in an appropriately lengthen- -
ed mixing part. For example, one of the components is provided as the product melt which
is mixed with the solvent before it is preferably supercooled and metered or sprayed into ; ~ ~
the high-speed mixer. ~ i
~' - ~;, ;", . '
'':. ~: ~' .'
